1. Field of the Invention
The present invention is generally related to nuclear reactor fuel assemblies and in particular to a thimble tube sleeve for reducing thimble tube wear.
2. General Background
Operating experience with typical unmodified and early model Westinghouse and Framatome/EDF nuclear reactors has shown that a high maintenance and wear problem exists in the guide path for the nuclear instrument thimble tubes which contain nuclear instruments and leads. The thimble tubes are guided into the inside diameter of the fuel assembly instrument sheath (tube) through guide paths located inside column structures that are integral parts of the reactor lower internals. The wear problem on the thimble tubes is attributable to the high velocity coolant flow along the length of the thimble tubes. The high velocity flow causes the thimble tubes to vibrate. The vibrating contact of thimble tubes against the lower core support plate, through which the thimble tubes pass, causes rapid and excessive wear of the thimble tubes. A known approach to alleviate the problem is the addition of sleeves anywhere from 30 to over 50 inches in length to house the thimble tubes above and below the core support plate. Although this has been somewhat effective in mitigating the flow-induced vibration, there still exists a need for a more effective means of further reducing or preventing flow-induced vibration of the thimble tubes.